I'm posting this for K0DD... email him at rbonner@qro.com
Class of operation is selected by BIAS.
Class AB1 and AB2 is just a drive issue. You can either run the tube closer
electrically to grid current or drive it higher and develop small amounts of
grid current.
The 4CX1000 they state in the example on the reflector allows 5ma of grid
current but zero grid dissipation. The 30S1 Collins ran AB2, cathode driven -
grounded grid with bias supplies?. You ran a small amount of positive grid
current (4ma max) at full power.
If the tube is designed to run AB2 you can do it without IMD coming out of your
ass.
Class A has less little IMD, class B has a lot? C would be really nasty if not
for output filtering (good tank circuit)
Plate efficiencies are progressively better by class. A 30%, B 66%, C upwards
of 75%... Class D 85% D is like digital switching of tubes? Class A is tube
conducts through 360 degrees of the amplifier cycle. Class B the tube conducts
through 180 degrees of the cycle and the collapse of the tank circuit voltage
fills in the second half? Class C the tank does what it can to fill in the back
half?
In audio or modulators designers are able to get the quality of class A by doing
a push-pull amplifier. This is a back to back set of class B amplifiers running
180 degrees from each other completing 360 degrees of the cycle. TA-DA.
Class C amps are only good on CW, FM and the final stage modulated AM. Either
plate, screen or suppressor modulated?. Class C amps are not LINEARS (no matter
how much a ham wants them to be) because the complete cycle is not reproduced.
This is why a 2 meter brick for FM use sounds like crap on SSB and you must
rebias the unit into a LINEAR range before it is usable on SSB.
On grid supplies. Control grid supply must be stable. Many guys use regulators
or like OA2 tubes in series to get bias? These are fairly high impedance. It
is necessary to have a medium to low impedance to ground to bleed off any
negative currents from secondary grid emissions on the control grid. This means
bleeder resistors of low impedance. Secondary emissions from the control grid
will cause the grid to draw current and the tube to take off in runaway.
The higher the grid circuit impedance (RF) the more unstable the amplifier
becomes. This is why many tube manufacturers (Svetlana) to recommend low values
of resistance to ground RF wise on the grid. It runs up the drive power but it
stabilizes the amplifier.
Any grounded screen or cathode RF amp with 50 OHMS to ground (RF) on the control
grid, across which the drive is applied wouldn?t even dream of oscillating.
Very stable situation. Change that to 10K and you might have a KW oscillator
that take no power to drive.
OK, screen supply. This supply must be of good design and of components capable
of handling the power required, plus it must be HEAVILY BLED. The heavy
bleeders are required to bleed off potential caused by secondary emissions from
the screen grid. These things spew electrons just like cathodes (though less)
and this emission causes the screen to build potential on the screen closer to
the anode. Very bad? Causes tube runaway. Fortunately screen supplies even in
megawatt tubes are still under a couple thousand volts. It is very easy to
build well designed supplies in this dissipation range. However skimp here and
say BYE-BYE to your tubey.
You must also never have a situation where screen voltage is on your tube
without Plate voltage. I would use the same fuses or breakers for both
supplies. Sub fuse the screen but make sure if you pop the plate and the screen
shuts off. You CAN supply drive and control grid bias to a tetrode without
screen and plate. You may never supply drive to a GG amplifier without plate
voltage.
Also NEVER under any circumstances break the cathode lead on any amplifier. The
ALPHA 77 amp used a relay to disconnect the cathode and select a fixed standby
bias supply. In the instance the cathode becomes disconnected the cathode tries
to hit anode voltage and things in the cathode circuit not designed for 4000
volts break. This is a very bad design. It worked sort of in the stock amp,
however once you start putting external transformers and raise the supply juice
the weakness reared its ugly head.
This also means CATHODE FUSES are bad. If you feel you must use one, make sure
it has a resistor across it. I always used 500 ohms at 25 watts. This will
bias the amp slightly on failure of the fuse. Eimac recommends 5000 Ohms. This
also means the plate current meter in th cathode lead will need a resistor
across it incase it opens up?
Anode supply, build away? Tetrode amps? I?d build a nice big choke input unit
that might lose maybe 5-10 % max under full load. 4100V then at 2 amps you
might have 3800V.. You have quite the supply there.
Grounded grid, go ahead and build a cap input jobby. But just remember the more
that supply moves around the more ALL parameters change on the tube operation..
The beefier supply will most likely be the cleaner amp if all other tube factors
are taken into account.
May I recommend all these guys read the BIBLE. Care and feeding of Power Grid
Tubes. It is available free of charge in PDF format on the Eimac (CPI) website.
Not only is it great reading for Poindexters, but it will help understand how
and why this stuff works.
Sincerely,
Bob Bonner KØDD
Sincerely,
Dr. William J. Schmidt, II K9HZ
Trustee of the North American QRO - Central Division Club - K9ZC
"Collector of Edison Wind-up Phonographs... Do you have one for me?"
Email: <mailto:bill@wjschmidt.com> bill@wjschmidt.com
Alternate Email: <mailto:wmschmidt@charter.net> wmschmidt@charter.net
WebPage: <http://www.wjschmidt.com/> www.wjschmidt.com
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